The invention relates to a high temperature and high pressure stripping and washing process which is excellent for use in separating portions of a feedstock between two high pressure reaction zones. More particularly, the invention relates to a high pressure, high temperature stripping and washing process which is well suited as an intermediate step in processes for treating Diesel and vacuum gas oil feeds so as to provide an FCC feedstock having reduced sulfur content and a Diesel fuel product having reduced sulfur content and enhanced cetane number.
Many refineries hydrotreat virgin and cracked feedstocks in order to obtain upgraded gasoline and Diesel products. These refineries utilize high-pressure units. High pressure hydrodesulfurization (HDS) units can be utilized with cracked vacuum gas oil (VGO), and when operated between 700-1200 psig, can achieve HDS conversion rates of greater than 99% so as provide a product having a sulfur content between 0.002 and 0.12% wt. This product can then be fed to a fluid catalytic cracking (FCC) process to produce gasolines and Diesel fuels with sulfur content less than 150 ppm and 600 ppm respectively. Unfortunately, the Diesel fraction produced in an FCC process from such a VGO feed typically has a cetane number of only about 20-30, which prevents this product from being incorporated into the Diesel pools. In order to be used, this Diesel fraction must be treated with additional hydrotreating steps. In addition, numerous other Diesel streams are readily available in the refineries such as straight run kerosene and Diesel, thermal cracked Diesel and the like, all of which have high sulfur content and typically medium cetane number that will require an additional deep hydrotreatment.
Conventional low-medium pressure Diesel hydrotreatment can satisfactorily reduce the sulfur content, but provides only small improvements in cetane number, in the range of 2-4 point increments.
Typical catalysts for use in hydrotreating to increase cetane number are extremely sensitive to even small amounts of sulfur, and therefore cannot readily be incorporated into an HDS reactor.
Alternatives for processing in order to attempt to address the sulfur and cetane number objectives include two-stage hydroprocessing. Unfortunately, conventional two-stage processing requires a separation to be carried out between the stages, and conventional separation processes are carried out at low temperature, low pressure, or both, resulting in the need for additional compression systems, one for each stage, which can double equipment and operation costs.
It is clear that the need remains for a method for treating VGO feedstocks and other Diesel feedstocks so as to advantageously reduce sulfur while improving cetane number. Further, the need remains for a process whereby separation of components is achieved at high temperature and pressure so as to avoid the need for additional compression equipment and the like.
It is therefore the primary object of the present invention to provide a process whereby VGO and Diesel feedstocks can advantageously and economically be converted into valuable end products.
It is another object of the invention to provide a process which can advantageously find use in revamping actual facilities or building new ones.
It is a further object of the invention to provide a process for high pressure and high temperature separation to produce an intermediate feedstock which can be blended with an external Diesel component to be sequentially treated in a Diesel hydrotreating stage.
Other objects and advantages will appear herein below.
In accordance with the present invention, the foregoing objects and advantages have been readily attained.
According to the invention, a process is provided for sequentially hydrotreating vacuum gas oil and Diesel, which process comprises the steps of providing a reaction feed containing vacuum gas oil, Diesel and sulfur-containing compounds; providing a stripping gas; providing a washing feed; and mixing said reaction feed, said stripping gas and said washing feed in a stripping and washing zone so as to obtain a gas phase containing said sulfur-containing compounds and a liquid phase substantially free of said sulfur-containing compounds, wherein said reaction feed is provided at a reaction feed pressure of between about 700 psig and about 1300 psig, and wherein said stripping and washing zone is operated at a pressure within about 50 psig of said reaction feed pressure.
The hydrodesulfurization and hydrotreating reactors, as well as the stripping/washing separator, are advantageously operated at substantially the same pressure, and preferably substantially the same temperature, thereby avoiding the need for additional compressor equipment between stages and limiting the need for additional heating between stages as well.